Carbon fibers have been used for many purposes in composite materials because of their exceedingly high tenacity. However, as is generally known, the adhesion between carbon fibers and resin matrices is not good. For this reason the surface of the carbon fiber is usually treated to increase the surface wetability or adhesion of the carbon fiber to the resin and to increase the interlaminar shear strength (ILSS) of the composite material.
Electrolytic oxidation of the carbon fiber surface is available and is extensively used for this purpose, but it is not without serious disadvantages.
The surface treatment of carbon fibers has heretofore been described in Japanese Patent Publication No. 29942/72, assigned to the assignee hereof. Although that publication describes an electrolytic treating bath including anodic terminals which are designed to permit the carbon fiber yarn to be fed to and to be taken from the bath without directly contacting the terminals, together with a means for continuously flowing and flushing the electrolyte through the anodic terminals, problems have been encountered in attempting to produce strong carbon fibers, having uniform strength along their lengths, utilizing carbon fiber surface treatments as disclosed therein, as will be developed further by reference to FIG. 1 of the drawings, which shows an anodic and cathodic arrangement of the type disclosed in the aforesaid Japanese Patent Publication. In FIG. 1, carbon fiber (CF) 1 taken off from a reel (not shown) is conducted onto an anode current input terminal 2 which is connected as anode to a current supply 3 and is supplied with anodic current. It then passes through an electrolytic bath 4. Cathode 5 in the electrolytic bath 4 is connected to the current supply 3 and is positioned beneath the running carbon fiber, which is kept substantially parallel to the cathode 5, and passes the second terminal 2 also connected as an anode to the current supply 3. The running fiber is then taken up on a reel (not shown).
In the foregoing procedure, the surface of the carbon fiber becomes the anode in the electrolytic bath 4 and is continuously oxidized by electrolysis. The electrical potential on the carbon fiber decreases as it moves away from the terminals 2 because of its rather high electrical resistivity compared with usual electric conductors such as metals and the like. The density of the current which flows out of its surface also decreases, and as a result, the carbon fiber in passing through the electrolytic bath does not undergo a uniform electrolysis and the effect of its surface treatment is variable.
The term "density of current flowing from the carbon fiber" as used herein means the density of current per unit length of carbon fiber and is represented by the following equation: ##EQU1##
In the surface treatment of carbon fibers, excessive current density causes deterioration of the tensile strength of the treated fiber. On the other hand, insufficient current density decreases the efficiency and the effectiveness of the surface treatment and adversely affects adhesion between the fiber and the resin matrix. Accordingly, problems have arisen, especially when a high current density was required, since this caused serious deterioration of tensile strength as a result of the electrolytic treatment. This has occurred, for example, in the case of tow type or woven structure carbon fibers which have higher density of fiber than the usual carbon fibers or graphite fibers which have more lubricity than the usual carbon fiber.
Efforts have been made to improve the unformity of the electrolytic treatment of carbon fibers and to overcome the disadvantages of non-uniformity by compensating or adjusting the potential and the current density of the carbon fiber running through the electrolytic bath, attaching plural terminals supplying anodic current, and applying uniformity of electrolysis. A typical example of such a suggestion appears in British Pat. No. 1,326,736, published Aug. 15, 1973, which indicates that current may be transferred to a carbon fiber by standard means such as by clips, or by passing the fiber through mercury, or over a metal or carbon or other conductive material roller which serves as the anode current input terminal. Similar comments and suggestions appear in the United States Patent to Chapman et al. U.S. Pat. No. 3,759,805, granted Sept. 18, 1973. However, proposals of this type have been found to be disadvantageous in that the act of increasing the number of terminals and the number of times the carbon fiber touches the terminals has tended to damage the carbon fiber surface mechanically. As a result, the carbon fiber has tended to develop considerable surface fuzz and to wind or bend itself around the terminals, seriously interfering with processability and with product quality.